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Versions: 00 01 02 03 04 05 rfc2718                                     
INTERNET-DRAFT                                           Larry Masinter
<draft-ietf-urlreg-guide-03.txt>                   Harald T. Alvestrand
August 7, 1998                                              Dan Zigmond
                                                             Rich Petke

                  Guidelines for new URL Schemes

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its
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   Distribution of this memo is unlimited.

   This Internet Draft expires February 7, 1999.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.


   A Uniform Resource Locator (URL) is a compact string representation
   of the location for a resource that is available via the Internet.
   This document provides guidelines for the definition of new URL

1. Introduction

   A Uniform Resource Locator (URL) is a compact string representation
   of the location for a resource that is available via the Internet.
   RFC [URI-SYNTAX] [1] defines the general syntax and semantics of URIs,
   and, by inclusion, URLs.  URLs are designated by including a
   "<scheme>:" and then a "<scheme-specific-part>".  Many URL schemes
   are already defined.

   This document provides guidelines for the definition of new URL
   schemes, for consideration by those who are defining and
   registering or evaluating those definitions.

   The process by which new URL schemes are registered is defined in

2. Guidelines for new URL schemes

   Because new URL schemes potentially complicate client software, new
   schemes must have demonstrable utility and operability, as well as
   compatibility with existing URL schemes.  This section elaborates
   these criteria.

2.1 Syntactic compatibility

   New URL schemes should follow the same syntactic conventions of
   existing schemes when appropriate.

2.1.1 Improper use of "//" following "<scheme>:"

   Contrary to some examples set in past years, the use of double
   slashes as the first component of the <scheme-specific-part> of a
   URL is not simply an artistic indicator that what follows is a URL:
   Double slashes are used ONLY when the syntax of the URL's
   <scheme-specific-part> contains a hierarchical structure as
   described in RFC [URI-SYNTAX].  In URLs from such schemes, the use
   of double slashes indicates that what follows is the top
   hierarchical element for a naming authority.  (See section 3 of RFC
   [URI-SYNTAX] for more details.)  URL schemes which do not contain a
   conformant hierarchical structure in their <scheme-specific-part>
   should not use double slashes following the "<scheme>:" string.

2.1.2 Compatibility with relative URLs

   URL schemes should use the generic URL syntax if they are intended
   to be used with relative URLs.  A description of the allowed
   relative forms should be included in the scheme's definition.
   Many applications use relative URLs extensively.  Specifically,

   o Can the scheme be parsed according to RFC [URI-SYNTAX] - that is,
     if the tokens "//", "/", ";", "?" and "#" are used, do they have
     the meaning given in RFC [URI-SYNTAX]?

   o Does the scheme make sense to use it in relative URLs like those
     RFC [URI-SYNTAX] specifies?

   o If the scheme syntax is designed to be broken into pieces, does
     the documentation for the scheme's syntax specify what those
     pieces are, why it should be broken in this way, and why the
     breaks aren't where RFC [URI-SYNTAX] says that they usually should

   o If the scheme has a hierarchy, does it go left-to-right and with
     slash separators like RFC [URI-SYNTAX]?  If not, why not?

2.2 Is the scheme well defined?

   It is important that the semantics of the "resource" that a URL
   "locates" be well defined.  This might mean different things
   depending on the nature of the URL scheme.

2.2.1 Clear mapping from other name spaces

   In many cases, new URL schemes are defined as ways to translate
   other protocols and name spaces into the general framework of
   URLs.  The "ftp" URL scheme translates from the FTP protocol, while
   the "mid" URL scheme translates from the Message-ID field of

   In either case, the description of the mapping must be complete,
   must describe how character sets get encoded or not in URLs, must
   describe exactly how all legal values of the base standard can be
   represented using the URL scheme, and exactly which modifiers,
   alternate forms and other artifacts from the base standards are
   included or not included.  These requirements are elaborated

2.2.2 URL schemes associated with network protocols

   Most new URL schemes are associated with network resources that
   have one or several network protocols that can access them.  The
   'ftp', 'news', and 'http' schemes are of this nature.  For such
   schemes, the specification should completely describe how URLs are
   translated into protocol actions in sufficient detail to make the
   access of the network resource unambiguous.  If an implementation
   of the URL scheme requires some configuration, the configuration
   elements must be clearly identified.  (For example, the 'news'
   scheme, if implemented using NTTP, requires configuration of the
   NTTP server.)

2.2.3 Character encoding

   When describing URL schemes in which (some of) the elements of
   the URL are actually representations of sequences of characters,
   care should be taken not to introduce unnecessary variety in the
   ways in which characters are encoded into octets and then into
   URL characters.  Unless there is some compelling reason for a
   particular scheme to do otherwise, translating character sequences
   into UTF-8 (RFC 2044) [3] and then subsequently using the %HH
   encoding for unsafe octets is recommended.

2.2.4 Definition of non-protocol URL schemes

   In some cases, URL schemes do not have particular network protocols
   associated with them, because their use is limited to contexts
   where the access method is understood.  This is the case, for
   example, with the "cid" and "mid" URL schemes.  For these URL
   schemes, the specification should describe the notation of the
   scheme and a complete mapping of the locator from its source.

2.2.5 Definition of URL schemes not associated with data resources

   Most URL schemes locate Internet resources that correspond
   to data objects that can be retrieved or modified.  This is the
   case with "ftp" and "http", for example.  However, some URL schemes
   do not; for example, the "mailto" URL scheme corresponds to an
   Internet mail address.

   If a new URL scheme does not locate resources that are data
   objects, the properties of names in the new space must be clearly

2.2.6 Definition of operations

   In some contexts (for example, HTML forms) it is possible to
   specify any one of a list of operations to be performed on a
   specific URL.  (Outside forms, it is generally assumed to be
   something you GET.)

   The URL scheme definition should describe all well-defined
   operations on the URL identifier, and what they are supposed to

   Some URL schemes (for example, "telnet") provide location
   information for hooking onto bi-directional data streams, and don't
   fit the "infoaccess" paradigm of most URLs very well; this should
   be documented.

   NOTE: It is perfectly valid to say that "no operation apart from
   GET is defined for this URL".  It is also valid to say that "there's
   only one operation defined for this URL, and it's not very
   GET-like".  The important point is that what is defined on this type
   is described.

2.3 Demonstrated utility

   URL schemes should have demonstrated utility.  New URL schemes are
   expensive things to support.  Often they require special code in
   browsers, proxies, and/or servers.  Having a lot of ways to say the
   same thing needless complicates these programs without adding value
   to the Internet.

   The kinds of things that are useful include:

   o Things that cannot be referred to in any other way.

   o Things where it is much easier to get at them using this scheme
     than (for instance) a proxy gateway.

2.3.1 Proxy into HTTP/HTML

   One way to provide a demonstration of utility is via a gateway
   which provides objects in the new scheme for clients using an
   existing protocol.  It is much easier to deploy gateways to a new
   service than it is to deploy browsers that understand the new URL

   Things to look for when thinking about a proxy are:

   o Is there a single global resolution mechanism whereby any proxy
     can find the referenced object?
   o If not, is there a way in which the user can find any object of
     this type, and "run his own proxy"?
   o Are the operations mappable one-to-one (or possibly using
     modifiers) to HTTP operations?
   o Is the type of returned objects well defined?
      - as MIME content-types?
      - as something that can be translated to HTML?
   o Is there running code for a proxy?

2.4 Are there security considerations?

   Above and beyond the security considerations of the base mechanism
   a scheme builds upon, one must think of things that can happen in
   the normal course of URL usage.

   In particular:

   o Does the user need to be warned that such a thing is happening
     without an explicit request (GET for the source of an IMG tag,
     for instance)?  This has implications for the design of a proxy
     gateway, of course.

   o Is it possible to fake URLs of this type that point to different
     things in a dangerous way?

   o Are there mechanisms for identifying the requester that can be
     used or need to be used with this mechanism (the From: field in a
     mailto: URL, or the Kerberos login required for AFS access in the
     AFS: URL, for instance)?

   o Does the mechanism contain passwords or other security
     information that are passed inside the referring document in the
     clear (as in the "ftp" URL, for instance)?

2.5 Does it start with UR?

   Any scheme starting with the letters "U" and "R", in particular if
   it attaches any of the meanings "uniform", "universal" or
   "unifying" to the first letter, is going to cause intense debate,
   and generate much heat (but maybe little light).

   Any such proposal should either make sure that there is a large
   consensus behind it that it will be the only scheme of its type, or
   pick another name.

2.6 Non-considerations

   Some issues that are often raised but are not relevant to new URL
   schemes include the following.

2.6.1 Are all objects accessible?

   Can all objects in the world that are validly identified by a
   scheme be accessed by any UA implementing it?

   Sometimes the answer will be yes and sometimes no; often it will
   depend on factors (like firewalls or client configuration) not
   directly related to the scheme itself.

3. Security considerations

   New URL schemes are required to address all security considerations
   in their definitions.

4. IANA considerations

   The process by which URL schemes names are registered is specified

5. References

   [1] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource
       Identifiers (URI): Generic Syntax", RFC [URI-SYNTAX], August 1998

   [2] Petke, R., "Registration Procedures for URL Scheme Names",
       RFC [URL-PROCESS], August 1998

   [3] Yergeau, F., "UTF-8, A Transformation Format of Unicode and ISO
       10646", RFC 2044, October 1996.

6. Authors' Addresses

   Larry Masinter
   Xerox Corporation
   Palo Alto Research Center
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Fax: +1-415-812-4333
   EMail: masinter@parc.xerox.com

   Harald Tveit Alvestrand
   Maxware, Pirsenteret
   N-7005 Trondheim
   Voice: +47 73 54 57 00
   EMail: harald.alvestrand@maxware.no

   Dan Zigmond
   WebTV Networks, Inc.
   305 Lytton Avenue
   Palo Alto, CA 94301
   Voice: +1-650-614-6071
   EMail: djz@corp.webtv.net

   Rich Petke
   WorldCom Advanced Networks
   5000 Britton Road
   P. O. Box 5000
   Hilliard, OH 43026-5000
   Voice: +1-614-723-4157
   Fax: +1-614-723-1333
   EMail: rpetke@compuserve.net